Microwave switches are being expected to perform at higher frequencies, with an improved ON to OFF ratio, a reduced insertion loss and increased isolation. A photo-conductive switch (PCS) uses light to control its electronic conductivity and therefore to modulate electronic signals passing through it. A photo-conductive switch has less stray electronic impedance than an equivalent electronically-controlled switch such as a transistor. Consequently, photo-conductive switches are potentially better suited for use as high-frequency and high-performance microwave switches.
U.S. Pat. No. 3,917,943 to Auston discloses a first type of PCS that is driven by an ultra-short optical pulse and is fabricated on a semiconductor substrate. Two gold micro-strip transmission lines separated by a narrow gap are located on the surface of a light-absorbing insulating semiconductor substrate. A first optical pulse directed to the substrate through the gap turns the PCS ON by generating copious electric charges on the substrate surface in the gap. A second optical pulse that begins during the first optical pulse and is directed to the gap generates copious electric charges in the bulk of the substrate extending down to the ground plane. This shorts the micro-strip transmission lines to ground, and switches the PCS OFF. The substrate is grown at a low temperature or is ion implanted to shorten the carrier lifetime to provide a very fast response. However, this also reduces the carrier mobility, which causes the PCS to have a high insertion loss.
U.S. Pat. No. 4,755,663 to Derkits, Jr. indicates that a disadvantage of the Auston PCS is that the electrical impulse created by the optical pulse is dominated by carrier recombination, rather than carrier transport. Derkits discloses a PCS in which the portion of the substrate constituting the gap includes a region composed of a textured-surface, graded-composition photosensitive semiconductor material. Illuminating the gap with a beam of light of sufficient intensity to generate charge carriers at the surface of the photosensitive semiconductor material causes the PCS to conduct.
FIG. 1 shows an embodiment of the PCS 1 disclosed by Derkits, Jr. In this, the semi-insulating semiconductor substrate 10, preferably of silicon, has the ground plane electrode 11 or an ohmic contact located on its major surface 21. On the opposite major surface 22 of the substrate is located the layer 18 of a wide band-gap energy semiconductor material. Overlaying the layer 18 is the layer 19 of a graded-composition alloy semiconductor material. Located on surface of the layer 19 and extending over part of the major surface 22 of the substrate are the electrodes 14 and 15 separated by the gap 13. The electrodes form the ohmic contact regions 17 with the layer 19.
The material of the layer 19 is an alloy of two semiconductor materials W and N. Semiconductor material W has a wide band-gap energy and semiconductor material N has a narrow band-gap energy. The fraction of the narrow band-gap energy semiconductor material N in the alloy increases monotonically with increasing distance from the layer 18 from a value of zero at the junction with the layer 18. The grooves 20 or other texturing are formed in the part of the layer 19 underlying the gap 13 to serve as charge separators.
Light falling on the layer 19 through the gap 13 creates charge carriers that provide electrical conduction between the electrodes 14 and 15. Extinguishing the light turns the PCS OFF by generating a quasi-electric field that sweeps the charge carriers into the region of the layer 19 where the narrow band-gap energy material is predominant. In this region, the grooves 20 separate the charge carriers and prevent further conduction between the electrode segments.
While the PCS disclosed by Derkits offers improved performance, PCSs with further performance improvements are required to meet the requirements of present-day technology.